1. Field of Invention
This invention relates to polymer-carbon sorbents suitable for removing heavy metals and toxic pollutants from flue gas, and adsorption of carbon dioxide. More specifically, this invention relates to a polymer-carbon adsorbent comprising a cured amine-containing polymer and a carbonaceous sorbent material to reduce emissions of elemental mercury and oxidized mercury and carbon dioxide from coal-fired power plants.
2. Description of the Related Art
Many heavy metals, especially mercury, are both hazardous and poisonous. Consequently, there is frequently a need to remove heavy metals, including mercury, from air streams around industrial processes such as chlor-alkali plants, iron ore processing, steel manufacturing, mining operations, and electronics manufacturing operations.
Mercury is a chemical of global concern specifically due to its long range environmental transport, its persistence in the environment once introduced, its ability to bio-accumulate in ecosystems, and its significant negative effects on human health and the environment. Mercury can be present in both liquid and gaseous waste streams. Mercury in gas streams provides additional challenges because of the volatility of metallic mercury and its compounds, which results in small quantities of mercury vaporizing from the heat of industrial processes, the burning of incinerator waste, and the burning of mercury-containing fuels.
Several approaches have been developed for effectively removing mercury species and other heavy metals from various streams. These overall approaches include, among others: liquid scrubbing technologies, homogenous gas-phase technologies, metal amalgamation techniques, and processes using various sorbent materials in different application schemes.
Capturing and isolating gaseous, elemental mercury from coal-fired power plants is a difficult technical problem because the gas volumes involved are great, the concentrations of mercury in the gas are low, and the gas temperatures are relatively high. Mercury typically exists as a trace element in coal, about 0.1 ppm by weight, although this can vary between coal types. As coal burns, the mercury volatilizes to form thermodynamically favored gaseous elemental mercury, Hg0. In the subsequent cooling of the combustion gases, interaction with other combustion products results in a portion of the elemental mercury being converted into gaseous oxidized form of mercury, Hg 2+2 and Hg+2 ions. Oxidation makes mercury easier to remove in a wet scrubber system, because most of the compounds formed from oxidized mercury are water-soluble, although toxic. The Hg0 is difficult to control, and is likely to enter the atmosphere because of its high vapor pressure and low water solubility. Small portions of Hg0, Hg2+2, and Hg+2 absorb onto residual particulates, such as fly ash, forming particle-bound mercury (HgP) that can be removed by filter or electrostatic precipitator.
A common practice for both gas and liquid removal of heavy metals is to contact the gas or liquid with a solid sorbent. “Sorbents” is a more general term used collectively for absorbents, which draw the heavy metal into their inner structure; adsorbents, which attract heavy metals and holding them to their surfaces; and chemisorbents, which form bonds between the surface molecules of the sorbent and the heavy metal species in a liquid or gas. Sorbents are typically in the form of particles, powders, or granules. Finely divided or microporous materials presenting large areas of active surface are strong adsorbents. Common adsorbents include activated carbon, activated alumina, and silica gel. Some sorbents, because of their size, shape, pore size, or chemical treatment, use more than one mechanism for removal of heavy metals. For example, some adsorbents may be treated or modified with materials, forming chemisorbents that will react with a heavy metal species.
Activated carbons are useful sorbents for sequestering mercury vapors in many applications, and have been studied extensively for use in flue gases. In small-scale gas processing, activated carbons may be used in fixed bed reactors or columns. However, for applications having large volumes of hot gas, such as coal-fired power plants, a fixed bed reactor or column may have cost issues associated with a large pressure drop, and maintenance of a fixed bed or column.
A number of inventive methods have been developed to apply mercury sorbent technologies to the large-scale gas streams of coal combustion for power generation. Moller et al., U.S. Pat. No. 4,889,698, and Chang, U.S. Pat. No. 5,505,766, for example, both describe the injection of fine powdered activated carbon (PAC) into flue gases at points along their journey through various pollution-control equipment trains. The PAC was then captured by a fabric filter. However, only about 15% of coal-fired boilers in the United States have such fabric filters, which allow for a high degree of mass transfer as the mercury-laden flue gas through a layer of the sorbent on the fabric filter bags. On the other hand, about 65% of United States coal-fired utility boilers have electrostatic precipitators (ESPs) instead of fabric filters, with no desulfurization systems for flue gases. An ESP configuration requires in-flight mercury removal, with some amount of time on the ESP plates parallel to the gas flow. That is, ESP configuration typically has less mass-transfer available to remove mercury vapor, compared to a flow of flue gas through a fabric filter.
Nelson, in U.S. Pat. No. 6,953,494, incorporated herein by reference, teaches a mercury-control method that can be applied to a number of combustion gas streams and a wide range of exhaust system configurations. Nelson teaches that activated carbon treated with bromine provides a more effective mercury sorbent material than untreated carbon or carbon treated with other halides. Bromine oxidizes the elemental mercury to toxic water soluble Hg+2 salt. Nelson's mercury treated activated carbon sorbent is especially suitable for in-flight removal of mercury. Nelson describes several configurations for use of in-flight removal of mercury that demonstrate the temperatures and contact times used in such processes.
FIG. 1 through 4 are schematic diagrams of exhaust gas systems describing example methods for using sorbents to remove and sequester mercury from hot combustion gases.
FIG. 1 shows an example system that applies mercury sorbents to a combustion gas stream where a fabric filter (baghouse) is utilized to collect fly ash generated during combustion. Coal, industrial wastes, or other fuels are combusted in a boiler 11 generating mercury-containing flue gas, which is cooled by steam tubes and an economizer 21. Flue gas typically then flows through ductwork 61 to an air cooler 22, which drops the gas temperature from about 300-to-400° C. down to about 150-to-200° C. and exits the air cooler in ductwork 62.
A mercury sorbent, stored in a container such as a bin 71, is fed to and through an injection line 72 to the ductwork 62 and injected through a multitude of lances to be widely dispersed in the hot combustion flue gas. Mixing with the flue gas, the sorbent adsorbs target heavy metal species, elemental mercury and oxidized mercury species from the flue gas. The sorbent flows with flue gas to a fabric filter 31 and is deposited on the filter bags in a filter cake, along with the fly ash and other gas-stream particulates. In the fabric filter the flue gas is forced through the filter cake and through the bag fabric. The flow of flue gas through the filter cake causes intimate contact between the sorbents and the remaining mercury in the flue gas, and will result in a high degree of mercury capture with a high degree of utilization of the sorbents. Cleansed of its mercury content and particulates, the flue gas exits the fabric filter 31 to ductwork 63, a smokestack 51, and then to the atmosphere. Upon cleaning of the fabric filter bags, the mercury sorbents in the filter cake fall into hoppers and are eventually emptied 81 from the fabric filter 31, and are disposed of along with the collected fly ash and unburned carbon. The mercury sorbents will generally make up on the order of 1 wt % of the collected particulates in pulverized coal power-plant applications.
FIG. 2 describes an example application of sorbents to a plant which has “cold-side” electrostatic precipitator (ESP) 32 instead of a fabric filter. Using an ESP provides a more difficult situation for mercury removal than with a fabric filter, because flue gas is not forced through the mercury sorbent in a filter cake layer of a collection bag. The hot mercury-containing combustion gas is generated in the boiler 11, as in FIG. 1, and flows through the same equipment to the ductwork 62. The mercury sorbent of bin 71 is similarly injected 72 into the ductwork to mix with the flue gas. Because of poorer mass transfer within the ESP 32, however, it is particularly important to inject at 72 as far ahead of any turning vanes, flow distributors, ductwork, and other exposed surface-area in the ductwork as possible. This not only provides more residence time for the sorbents to mix with and remove mercury from the flowing gas, but provides for more mass transfer area for the sorbent to collect on, further increasing the overall mass transfer and mercury removal. In the ESP 32, the sorbents are collected on plates with the fly ash and upon rapping of the plates are eventually discharged 81 from the ESP 32 for disposal along with the rest of the particulates.
Several variations on arrangements of FIGS. 1 and 2 might be suggested, based on a configuration of existing air pollution control equipment. For example, a wet scrubber for flue gas desulfurization could appear at 63 in FIGS. 1 and 2 or a particulate scrubber could replace ESP 32. Selective catalytic reduction (SCR) units for NOx reductions, which also can reduce Hg+2 to elemental mercury or flue gas conditioning systems to improve particulate removal, could also be placed in the equipment arrangements. Similarly, mercury sorbents could be injected while mixed in with sorbents for other flue gas components, such as calcium or magnesium hydroxide or oxide for flue gas SO3, HCl, or SO2, rather than injected alone. Alternately, the mercury sorbents could be injected in liquid slurry, which would quickly evaporate in the hot flue gas.
FIG. 3 applies the sorbents in a TOXECON® arrangement, a process patented in U.S. Pat. No. 5,505,766, and marketed by Electric Power Research, Inc., Palo Alto, Calif. Mercury sorbents 71 are injected after an ESP 32 into almost particulate-free ductwork 67 before a small, high-velocity fabric filter 33. In this manner the fly ash 80 does not become mixed with the carbonaceous sorbents, allowing the fly ash to be sold for concrete use. Moreover, the filter cake of fabric filter 33 would predominantly be mercury sorbent, allowing a longer residence time, higher utilization levels, and the possibility of recovering and re-injecting the sorbent to lower costs.
FIG. 4 illustrates sorbent usage at plants that have spray dryers for acid rain control. A mercury sorbent could be injected 62 before the spray dryer 41, into the spray dryer 41, into the ductwork 68, between the spray dryer and the particulate collector 31 or 32, or mixed in with the scrubber slurry itself.
Mercury has a high affinity for sulfur. Elemental mercury, in the presence of sulfur, readily forms mercury (II) sulfide when heated. Mercury (II) sulfide can exist in two chemically stable forms: a red, hexagonal complex (cinnabar), and a black metastable structure (metacinnabar). Mercury also readily forms complexes with other sulfur compounds, including sulfates (HgSO4), dithiocarbamates (Hg(Et2DTC)2) and various thioethers (Hg(SR)2). The affinity of mercury for sulfur has lead to many studies of sulfur-treated carbon adsorbents for the removal of mercury. See, for example, Bylina et al., Journal of Thermal Analysis and calorimetry (2009), 96(1), pp 91-96 “Thermal analysis of sulfur impregnated activated carbons with mercury absorbed from the Vapor Phase”; and Skrodas et al., Desalination (2007), 210(1-3), 281-286, “Role of activated carbon structural properties and surface chemistry in mercury adsorption.”
Sorbents in liquid systems typically include those with ionic groups to capture materials in solution. The ionic groups may be inherent in the sorbent material, or added through a treatment of another sorbent such as activated carbon. Materials such as amines and polyamines have been studied for use in removing metal ions. Polyamines are organic compounds that contain two or more primary amino groups. Polyamines generally have cations that are found at regularly-spaced intervals (unlike, say, Mg++ or Ca++, which are point charges).
Amines reacted with activated carbon have been studied for use in purifying water. Akio Sasaki, in U.S. Pat. No. 4,305,827, also teaches an adsorbent, obtained by reacting active carbon with a water-soluble amine and carbon disulfide, in the presence of water. The adsorbent is useful in removing heavy metals, especially mercury, silver, gold, copper, and cadmium from water. The preferred amines are divalent or polyvalent amines, including aromatic amines and poly(ethyleneimine). The adsorbents hold their adsorptive function well after being washed. Sasaki proposes that the amines react on the surface of the activated carbon; however, recently, there is some question that this occurs.
Sasaki, et al. studied a sorbent formed by reacting polyamines with CS2 in water, in the presence of palm-shell activated charcoal for used in removing Hg+2 ions in water. [See Sasaki, Akio, Kimura Yohiharu, Nippon Kagaku Kashi, 12, 880-886, (1997), “Preparation of polythiourea-immobilized activated charcoal and its utilization for selective adsorption of mercury(II) ion. Studies on functionalization of polymers by reactive processing. Part 5.”]. A Sasaki et al. propose that the secondary amine groups in the polymer backbone react with carbon disulfide to form thiourea crosslinked sites [e.g., >N—C(S)—N<].
Amine-containing polymers have been studied as sorbent materials for treatment of water. Some amine-containing polymers can be derived from natural sources. For example chitosan, is a de-acylated derivative of chitin, a glucosamine polysaccharide, is found in the shells of crabs, lobsters, and beetles. Chitosan has been used to absorb heavy metals from water and industrial waste streams. [See Hawley's Condensed Chemical Dictionary, 11th edition (1987).]
Masari, et al., in U.S. Pat. No. 4,125,708, describes the use of chitosan, modified with an anionic agent and glutaraldehyde, for removing superoxy-anion-forming ions, such chromium. The anionic agent is selected from sulfite, sulfate, chloride, hexafluoride, and borate groups. The glutaraldehyde serves as a crosslinking agent. Other crosslinking agents taught are glyoxal, glutaraldehyde, and dialdehyde starch. The crosslinked, anionically modified, nitrogen-containing product exhibits increased stability and insolubility over the non-crosslinked product. Masari et al. teach that such sorbents could be used by adding a filter to an already-existing industrial or municipal water purification system.
For mercury removal, use of a sulfur cure system would be attractive for polymer-carbon sorbents used in mercury removal because sulfur has an affinity for mercury. Sulfur vulcanization of polymers is used as a conventional curing system for strength and shelf life of rubber. In addition, sulfur vulcanization technology allows for a range of vulcanization speeds and elastomer properties. In vulcanization of poly(isoprene) rubber, for example, sulfur forms a bond at points of unsaturation in the polymer, forming crosslinks between the polymer chains. In these cases, the elastomer molecules must contain allylic hydrogen atoms. For additional information on curing systems see, for example, The Science and Technology of Rubber, (1978), edited by Frederick R. Eirich, Academic Press, New York, “Vulcanization” by A. Y. Coran, pp 291-338.
Sulfur vulcanization is usually performed with an accelerator to control the cure time and characteristics. Accelerators include a number of sulfur-containing compounds, plus a few non-sulfur types, such as ureas, guanidines, and aldehydeamines. Accelerated sulfur vulcanization has been extended to other diene synthetic rubbers, such as SBR, butyl and nitrile rubbers. Accelerators that do not decompose or react with olefins at curing temperatures require an activator selected from basic metallic oxides or salts of lead, calcium, zinc, or magnesium. Some accelerators such as zinc salts of mercaptobenzothiazole and the dithiocarbamic acids do not require an additional activator. [See, for example, Textbook of Polymer Science, 2nd Ed., Fred W. Billmeyer.]
Also the continuous rise of the atmospheric carbon dioxide concentration and its link with climate change demand a technological solution. This solution is especially needed for industries where large amounts of carbon dioxide result from burning operations, such as utility companies where coal is the fuel source. Carbon capture and sequestration to reduce such emissions have been considered for such mitigation. However, many industries use an amine-scrubbing technique as their solution; however, these methods cost can be very high. [See for example, R. S. Haszeldine, Science 325, 1647-1652 (2009).] Recently carbon-based support materials, such as PEI on carbon materials, have become of interest. [See, for example, D. Wang et al., Energy Fuels 25, 456-458 (2011).]
Therefore, a better system of polymer-carbon sorbents suitable for removing carbon dioxide, toxic materials and heavy metals from flue gas that is economical to run, especially on a commercial scale, is needed.